Plasma X-ray source

Phys. Rev. Lett. 115, 083901 (2015)

Soft X-rays have long been an important tool for investigating the properties of material and biological systems and can reveal information about their electronic and magnetic characteristics. To conduct such research, coherent circularly polarized soft X-rays are often desired. However, the generation of circularly polarized soft X-rays is typically limited to a handful of large facilities. There are also emerging laboratory-scale techniques, in particular related to high-harmonic generation, but these sometimes exhibit low photon yields, making single-shot experiments troublesome. Now, Adrien Depresseux and colleagues from France, Czech Republic and Korea have experimentally demonstrated a 32.8 nm wavelength, plasma-based soft X-ray laser offering circular polarization. The laser-driven source outputs a Gaussian beam profile with 10¹⁰ photons per pulse, which the team notes should be practical for laboratory experimentation. To achieve this, a 1.36 J, 30 fs laser pulse pumps a krypton gas cell with an intensity of ∼10¹⁸ W cm⁻² at a focal point, creating a 5-mm-long column of ionized plasma. A second laser beam, with an energy of 16 mJ per 350 fs pulse is optimized to generate a 32.8 nm harmonic. The divergence of the beam is about 1 mrad and the pulses are a few picoseconds in duration. The polarization of the high harmonic is converted from linear to circular using a four-reflector polarizer set-up with an efficiency of 1.5% at 32.8 nm.